As highlighted in the part one of this post , one of the common challenges with GIS vector layers from different sources is getting them to align properly on the same georeferenced raster image.
In the part one of this post, we worked from ArcMap to QGIS i.e. we used an image georeferenced in ArcMap as a base map for drawing vector layers and then exported it for use also in QGIS. In this Part Two, we will be working with image georeferenced in QGIS Desktop and will see how that will go.
1. Launch your QGIS Desktop and on the main menu bar, go to Raster – Georeferencer – Georeferencer. We will be using this tool to georeference the image. Import the image in and georeference it. (Georeferencing Raster Images in QGIS has been covered in an earlier post)
2. Once you have finished georeferencing, you should notice that the default coordinates used is in WGS84 SRID 4326. Now we move to save the georeferenced image as a GeoTIFF file. We right click on the image layer and select ‘Save As’ in the dropdown list.
3. In the ‘Save Raster layer as’ dialog window, make sure you set the Format to gTiff option (A ) and change the default CRS to the one appropriate for the data (B ). For our demo, we will change from default CRS to use 4263 Minna. We are saving our GeoTIFF image with name- qgismap.
5. Now let us load the GeoTIFF file as a raster image into ArcMap 10x. First we should prepare the Data Frame to have the same CRS as the raster layer being imported. Right Click on Layers – Properties
6. The ‘Properties’ dialog window will open. Choose ‘Coordinate System’ tab and enter the same CRS as that for the GeoTIFF file.
7. Click on Add data to load the GeoTIFF file onto the map canvas.
8. The GeoTIFF file is now added to the map canvas.
9. Now we go back to our QGIS and start drawing vector layers. Click on Layer on the main menu, select Create Layer- New Shapefile Layer.
10. We create a shapefile and name it-racecourse. Note that when creating the shapefile, assign the same Coordinate Reference System (CRS) as that of the base map. Next we draw a polygon feature and save the editiing work.
11. To confirm proper alignment, we go back to ArcMap and use our catalog to navigate to the location where the shapefile-racecourse was saved.
13. Add the shapefile to the layer window. Result as seen on the screenshot below shows it aligns in ArcMap.
14. Now let us carry out the converse of what we just did and see whether the same result will be achieved. Right Click on your working folder – New – Shapefile. We will creating a shapefile in ArcMap and drawing a feature with it.
15. We digitize a line feature on the image and save the edits.
16. Go back to QGIS and navigate to where the shapefile .(linefeature3) that was just created was saved. Double click on it to add it as an active layer on the map canvas. Follow the red arrow and notice that the vector layer created in Arcmap aligns very well in QGIS environment.
*Contrary to my usual practice of making available a sample data for our tutorials, we are constrained from sharing this map because of rights and privileges that the National Lands and Survey Office has over it.
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